Closed cycle cryogen recirculation system and method

ABSTRACT

There is provided refrigeration system ( 1 ) and method for remote cooling of a thermal load having a first portion ( 27 ) and a second portion ( 25 ). The system comprises a cold source ( 4 ) having a first cooling stage ( 5 ) and a second cooling stage ( 6 ), the temperature of the first cooling stage being higher than the temperature of the second cooling stage. The system also comprises a cryogen circuit for circulation of a cryogen flow in a closed cycle, the closed cycle being thermally coupled to the cold source. The system further comprises a compressor ( 7 ) for compressing and circulating the cryogen flow in the cryogen circuit. The cryogen circuit comprises a first conduit for thermally connecting the first cooling stage of the cold source to the first portion of the thermal load so as to cool said first portion towards the temperature of the first cooling stage, and a second conduit for thermally connecting the second cooling stage of the cold source to the second portion of the thermal load so as to cool said second portion to wards the temperature of the second cooling stage. The cryogen flow in the system is a sub-cooled or saturated liquid, two phase, saturated or overheated, supercritical gas helium flow.

RELATED APPLICATIONS

The present application is a national stage application under 35 U.S.C.§371 of International Application No. PCT/EP2015/079195, filed 10 Dec.2015, which claims priority to European Patent Application No.14197216.6, filed 10 Dec. 2014. The above referenced applications arehereby incorporated by reference into the present application in theirentirety.

FIELD OF THE TECHNOLOGY

The present disclosure relates generally to the field of cryogeniccooling systems, and more particularly, to small-scale and low coolingcapacity cryogenic cooling systems which are suitable for coolingobjects which are located remotely, such as, but not limited to,superconducting magnets.

BACKGROUND

Large scale and large capacity cryogenic cooling systems, such as heliumrefrigerators and liquefiers are well known in the art.

For small-scale low-capacity cryogenic cooling systems, with coolingpower in the Watt-range at temperatures below 5K, cryocoolers arecommonly used. They traditionally consist of a compressor, a cold headand wherein the cold head contains one, two or maximum three coolingstages, with more or less predetermined temperature ranges. Due to theirdesign, systems based on cryocoolers are traditionally closed cyclesystems which are called “cryogen free” as they provide only coldsurfaces for the cooling of thermal loads.

Such cryocoolers are typically used for direct contact cooling oflaboratory-sized objects to be cooled down and maintained at the desiredtemperature, such as samples, superconductors, etc. Other applicationsinclude re-condensation of vaporized fluids such as helium at 4.5K,hydrogen at 20K or nitrogen at 70K.

This results in a significant temperature gradient within the object tobe cooled, which means that the cryocooler needs to be at asignificantly lower temperature than the desired load temperatureresulting in a lower efficiency of the system and lower cooling power.

To avoid temperature gradients within the object to be cooled, bathcooling is a well-known solution and this is still commonly performed incryogenic laboratories in universities and research centres. Thissolution was also used for first-generation MRI magnets. Such systemsare open-cycle, and therefore require a storage dewar which has to berefilled or changed on a regular basis, and thus requires additionalhandling of a hazardous cryogenic fluid. In cases where the requiredperformance is achievable within the range of cryocoolers, cryocoolersare well adapted to re-condensate the vaporized liquid.

However, where bath cooling is not possible or where remote cooling isrequired, such as when the cryocooler cannot be located in sufficientproximity to the load for any reason, state of the art cryocoolers arenot suitable for such cooling purposes.

There is therefore a need for a compact refrigeration system that iscapable of providing low-capacity refrigeration for remote cooling of aload. The inventors of the present invention have addressed the aboveshortcomings of conventional low-capacity cryogenic refrigerators, aswill be explained below.

SUMMARY OF INVENTION

According to one aspect of the present invention, there is provided arefrigeration system for remote cooling of a thermal load having a firstportion and a second portion, the system comprising: a cold sourcehaving a first cooling stage and a second cooling stage, the temperatureof the first cooling stage being higher than the temperature of thesecond cooling stage; a cryogen circuit for circulation of a cryogenflow in a closed cycle, the closed cycle being thermally coupled to thecold source; and a compressor for compressing and circulating thecryogen flow in the cryogen circuit, wherein the cryogen circuitcomprises a first conduit for thermally connecting the first coolingstage of the cold source to the first portion of the thermal load so asto cool said first portion towards the temperature of the first coolingstage, and a second conduit for thermally connecting the second coolingstage of the cold source to the second portion of the thermal load so asto cool said second portion towards the temperature of the secondcooling stage, and wherein the cryogen flow is a sub-cooled or saturatedliquid, two phase, saturated or overheated, supercritical gas heliumflow. It is especially interesting to operate the system of the presentinvention with a two phase cryogen flow, as the refrigeration using thelatent heat of the fluid is advantageous, because as the liquidevaporates the fluid does not fluctuate in temperature, thus allowing auniform temperature distribution of the fluid at the thermal load.Because of this uniform temperature distribution, the conventionalsolution to the problem as identified in the Background, of operatingthe cold source at a lower temperature than is desired for the load isno longer required, resulting in a higher cold source performance. Someadvantages of the refrigeration with two phase flow include the reducedmass flow in comparison to a single phase flow, and also that the twophase fluid may be returned from the thermal load at the sametemperature (thus reducing hotspots).

Consequently, and in contrast to the refrigeration systems using thesensible heat of the fluid, using the latent heat requires a lower massflow at a given cooling capacity.

Notwithstanding the above, the system of the present invention can alsooperate in single phase conditions using the latent heat of the cryogen.

The system of the present invention is the combination of a cold sourceand a cryogen circulation system; it provides a low capacity (forexample, 1 or 2 Watts at 4.5 K) refrigeration system with a cryogen massflow at low temperatures in a closed cycle at mass flow rates in theorder of magnitude of <0.1 g/second for two phase flow or up to 1g/second for single phase gas or supercritical helium flow. The closednature of the cryogen cycle allows for non-stop refrigeration, i.e. thesystem does not need to be stopped in order to change or re-fill thecryogen. Further, the system advantageously does not suffer from thedrawbacks of contact cooling, for example, temperatures gradients acrossthe load, punctual cooling or elaborated thermal anchoring.

The refrigeration system may preferably comprise at least one heatexchanger, which is used to thermalize the cryogen to at least one ofthe cold source stages and to exchange the enthalpy respectively betweenthe go and return streams of the cryogen.

The compressor of the refrigeration system may be a circulation pump atroom temperature. The advantage of using warm pumps with respect to anyother solution is that the compression work can be directly extracted tothe ambient environment by any common means, such as air or water heatexchangers. This is especially important when high compression ratiosare required, for example, for a Joule-Thomson expansion step.

In addition, the system of the present invention can provide one or two(or in some cases more) cryogen flows at different temperatures at thesame time. In other words, the cryogen flows in a single circuit but canbe taken at two different stages of the refrigeration processsimultaneously. The same cryogen can therefore be used at two differenttemperature levels.

Some applications of the system of the present invention include lowcapacity cryogenic refrigeration where contact cooling with a commoncold source is not suitable, where re-filling the cryogen source is notan option (for example, in a dewar), where re-condensation of the usedcryogen must be integrated into the system, where use of an immersioncryostat is not suitable, and where the available space next to thethermal load presents some constrains (for example available space orradioactive environment). Other uses may include recuperative cycles,multistage cascade cooling, helium condensation, helium liquefaction andremote cooling.

Preferably, the first portion of the thermal load could be a thermalshield, and said thermal shield may be actively cooled. The cryogen atthe higher temperature (e.g. 60 K) can be used to cool, for example,said active thermal shield. Preferably, the second portion of thethermal load could be a superconducting magnet. The cryogen at the lowertemperature (e.g. 4.2 K) could be used to cool, for example, saidsuperconducting magnet. When operating with two phase helium, theconfiguration of the refrigeration system of the present invention wouldprovide a uniform temperature profile at the second portion of thethermal load. This is particularly advantageous in the case where thesecond portion of the thermal load is a superconducting magnet coil andit is important that a uniform temperature profile with a lowtemperature gradient is achieved on the coil.

Each of the first conduit and the second conduit may preferably belocated within a transfer line, and preferably said transfer line haslow thermal losses. An advantage of the remote set-up of the “active”parts of the system (the cold source and the compressor) away from the“passive” parts of the system (the thermal load e.g. the superconductingmagnet) is that no harmful vibrations are transferred to the thermalload. In addition, said “active” parts are spatially separated from thethermal load. This is advantageous in case the load is in an environmentwith special constraints (for example radioactivity or limited spaceavailability).

Preferably, the first and second cooling stages of the refrigerationsystem and the first and second conduits of the closed cycle cryogencircuit may all connected in series. With respect to prior art systemsusing parallel cryogen circuits, the arrangement of the system of thepresent invention avoids the need for elaborate and expensive cryogeniccontrol systems and equipment.

The cold source of the refrigeration system may preferably be, but isnot limited to, a cryocooler. Any cold source providing a continuouscooling performance at the desired temperature would be suitable.However, cryocoolers exhibit advantages which make them particularlyadapted to the present invention: they are commercially available, easyto use and being cryogen-free, cryocoolers obviate the need for acryogen flow. This therefore also reduces the complexity of the system,and additionally limits the need for controls and instrumentation. Inits simplest form, the system could be implemented completely withoutcontrols (as per FIG. 1 and FIG. 2). Such a system is very robust andoffers a turn-key solution to users which therefore also do not need tobe particularly trained in cryogenics. In more sophisticatedimplementations (e.g. FIG. 3), a limited set of controls andinstrumentation can be used, increasing the versatility and flexibilityof the system, which would be particularly suited for the cooldown oflarger cold masses.

Preferably, the cold source may be contained within a cryostat or vacuumvessel for insulation purposes. Said cryostat or vacuum vessel isseparate and independent from a cryostat of the thermal load(s).Preferably, the cryostat or vacuum vessel may comprise an activelycooled thermal shield to reduce undesired thermal radiation to thecolder components.

Preferably, the system may further comprise a Joule-Thompson (JT)expansion step, which may be a JT pipe. Due to the JT-expansion, thefinal cryogen temperature at the thermal load is lower than afterthermalizing at the second stage of the cold source. Therefore, the coldsource can operate at a higher temperature than the final cryogentemperature. Since the cold source performance increases when working ata higher temperature, the overall cooling capacity would thereforeincrease which may lead to an overall higher cooling capacity of thecold source.

Preferably, depending on the construction of the system, it may act as arefrigerator or as a liquefier. For example, gas helium (GHe) may beliquefied to liquid helium (LHe). However, the main design objective ofthe present invention is to achieve a refrigeration system rather than aliquefaction system.

According to another aspect of the present invention, there is provideda method for cooling remotely cooling a thermal load using arefrigeration system, the method comprising: selecting the temperatureof a first cooling stage of a cold source of the system to be higherthan the temperature of a second stage of cooling of the cold source;circulating a cryogen flow in a closed cycle around the cryogen circuitof the refrigeration system, the closed cycle being thermally coupled tothe cold source, the cryogen circuit comprising a first conduit forthermally connecting the first cooling stage of the cold source to afirst portion of the thermal load so as to cool said first portiontowards the temperature of the first cooling stage, and a second conduitfor thermally connecting the second cooling stage of the cold source toa second portion of the thermal load so as to cool said second portiontowards the temperature of the second cooling stage; and compressing theflow in the cryogen circuit using a compressor of the refrigerationsystem, wherein the cryogen flow is a sub-cooled or saturated liquid,two phase, saturated or overheated, supercritical gas helium flow.

BRIEF DESCRIPTIONS OF DRAWINGS

Certain embodiments of the present invention will now be described byway of example only with reference to the accompanying drawings, inwhich:

FIG. 1 is a schematic diagram of a refrigeration system and a load to becooled according to the present invention;

FIG. 2 is a schematic diagram of a refrigeration system according to thepresent invention; and

FIG. 3 is a schematic diagram of another refrigeration system accordingto the present invention.

DETAILED DESCRIPTION OF DRAWINGS

Reference will now be made to FIG. 1, which is a schematic diagram of asystem according to the present invention. FIG. 1 shows a refrigerationsystem (1) and a thermal load (25, 27) to be cooled, each beinginstalled separately in two separate cryostats (2) The cold source (4),which in this case may be a cryocooler, cools the cryogen to therequired temperature, and then the cryogen is delivered to the thermalshield (25) and to the load (27) through a pipe system (26, 28). Alow-loss transfer line (24) is also shown.

FIG. 2 illustrates the refrigeration system (1) in more detail. This isone of the many possible configurations, which can vary according to thecooling requirements. This particular configuration could be consideredas the baseline of more complex configurations potentially resulting inhigher performances. An example of those more complex configurations ispresented in FIG. 3. The refrigeration system (1) in FIG. 2 consists ofa pre-cooling refrigerator and a cryogen circuit. The two systems arethermally coupled to each other. Most of the components are inside avacuum chamber or cryostat (2) and, where necessary, some of them arealso thermally protected by an actively cooled thermal shield againstundesired thermal radiation (3).

The pre-cooling refrigerator consists in this case of a commercial2-Stage (5, 6) cryocooler (4) (GM-type, Stirling type, Pulse Tube type,or other) or any other suitable refrigeration system. The first coolingstage (5) has a temperature that is higher than the temperature of thesecond cooling stage (6).

There is also shown in FIG. 2 a compressor (7) (or gas pump) at roomtemperature, a pipe system (8, 9) for the cryogen and various heatexchangers (10, 11, 12, 13, 15, 16, 17).

The cryogen circuit starts with a compressor (7) at room temperature,which is used to circulate the cryogen. The cryogen circulates through apipe system, which consists of the feed pipeline (8) and the returnpipeline (9). The feed pipeline (8) starts in the flow direction at thecompressor (7), goes through different heat exchangers (10, 11, 12, 15,16, 17) and ends at the feed through connection (23) to the thermalload. Correspondingly, the return pipeline (9) starts at the feedthrough connection (23) from the thermal load, goes through differentheat exchangers (13, 12, 11) and ends at the compressor (7).

The compressor (7) compresses and moves the cryogen gas through piperuns. The first heat exchanger (10) re-cools the cryogen gas back fromroom temperature. Subsequently a group of heat exchangers (11-13)transport the cryogen heat from the feed pipeline (8) to the returnpipeline (9). The cryogen is additionally cooled in the feed pipeline(8) by the cryocooler (4) at the heat exchangers on the first (15, 16)and second stage (17). After the cryogen is being pre-cooled in the feedpipeline (8), it flows towards the feed through connection (23) to thethermal load.

The cryogen can be taken at two different stages of the refrigerationprocess simultaneously, allowing its use at least at two differenttemperatures, or in case of need, at intermediate temperatures attemperature levels between ambient and first stage, or temperaturesbetween first and second stage. Additional pipes and connections may beneeded for this functionality. The following description focuses on thetwo stage configuration, however, other configurations comprising morestages are also conceivable. The cryogen at a higher temperature will bedelivered over a pipe system to the thermal shield (25) of the load,while the colder cryogen will go to the load (27). In both cases, thecryogen will return to the refrigeration system (1) after being used inorder to provide a closed cycle system.

Depending on the configuration of the refrigeration system (1) and thechosen cryogen, the system can be set up to provide a mass flow of acryogen as any one of a sub-cooled or saturated liquid, two phase,saturated or overheated, supercritical gas helium flow. Therefore, thesensible heat (the cryogen is gaseous or supercritical and changes itstemperature at the load to be cooled) or the latent heat (the liquidcryogen evaporates and therefore it does not change his temperature) ofthe cryogen can be used for refrigeration. After the cryogenrefrigerates the load it returns to the compressor (7) through thereturn pipeline (9), thus closing the cycle.

FIG. 3 illustrates the refrigeration system (1) in a more complexconfiguration and potentially with a higher performance than in FIG. 2.As mentioned before, the many possible configurations of the system canvary according to the cooling necessities. However, some basicprinciples presented in FIG. 2 remain the same in FIG. 3. In thisparticular case, the refrigeration system (1) consists of a pre-coolingrefrigerator, a cryogen circuit comprising a Joule Thompson (JT)expansion step and a transfer line (24). Most of the components areinside a vacuum chamber or cryostat (2) and some of them are alsothermally protected by an actively cooled thermal shield (3) againstundesired thermal radiation.

The pre-cooling refrigerator consists in this case of a commercial2-Stage (5, 6) cryocooler (4) or any other suitable refrigeration system(see above). The JT refrigerator includes a compressor (7) at roomtemperature, a pipe system (8, 9) for the cryogen, various heatexchangers (10-17) and a Joule Thompson (JT) expansion device (18). Inaddition, in order to reduce flow impedances and to accelerate cooldown, several bypass valves (19-22) can be installed in case ofnecessity.

The JT refrigerator starts with a compressor (7) at room temperature,which is used to compress and circulate the cryogen. The cryogencirculates through a pipe system, which consists of the feed pipeline(8) and the return pipeline (9). The feed pipeline (8) starts in theflow direction at the compressor (7), goes through various heatexchangers and valves (10-12, 15, 23, 16-18) and ends at the feedthrough connection (23) to the load. Correspondingly, the returnpipeline (9) starts at the feed through connection (24) from the load,goes through different heat exchangers (14-11) and ends at thecompressor (7).

For remote cooling, a transfer system is required. This may comprise anoptimized transfer line (24) which delivers the cryogen to the thermalloads. The optimization consists of minimizing the losses on all lines,and in particular in the cold circuit. This allows a more efficientintegration into the overall system.

After the compression of the cryogen gas, the first heat exchanger (10)cools the cryogen back from room temperature. Afterwards a group of heatexchangers (11-14) transport the cryogen heat from the feed pipeline (8)to the return pipeline (9). The cryogen is additionally cooled in thefeed pipeline (8) by the cryocooler (4) at the heat exchangers on thefirst (15, 16) and second stage (17). After the cryogen is beingpre-cooled in the feed pipeline (8), it flows through the JT expansiondevice (18).

The cryogen can be taken at two different stages of the refrigerationprocess simultaneously, allowing his use at two different temperatures.The cryogen at a higher temperature will be delivered over a pipe systemto the loads thermal shield (25), while the colder cryogen will go tothe load (27). In both cases, the cryogen will return to therefrigeration system (1) after being used, in order to provide a closedcycle system.

Depending on the configuration of the refrigeration system (1) and thechosen cryogen, the system can be set up to provide a mass flow ofeither sub-cooled or saturated liquid, two phase, saturated oroverheated, supercritical gas. Therefore, the sensible heat (the cryogenis gaseous and changes his temperature) and/or the latent heat (theliquid cryogen evaporates and therefore it does not change histemperature) of the cryogen can be used for refrigeration. After thecryogen refrigerates the load it returns to the compressor (7) throughthe return pipeline (9), closing the cycle.

When the entire system is switched on, the cryogen starts circulatingand the cryocooler (4) starts cooling down from room temperature. Duringthis period of time, the bypass-valves can be opened (19-22) in case ofnecessity. Consequently, the cryogen can bypass some components (13, 14,18), reducing the overall pressure losses. Once the cool down process isfinished, the bypass-valves (19-22) can be closed again.

KEY

-   -   1. Refrigeration system    -   2. Cryostat or vacuum chamber    -   3. Thermal Shield of the Cryogen Recirculation System    -   4. Cryocooler    -   5. First stage of cryocooler    -   6. Second stage of cryocooler    -   7. Compressor    -   8. Feed pipeline    -   9. Return pipeline    -   10. Heat exchanger 1 (HX1)    -   11. Heat exchanger 2 (HX2)    -   12. Heat exchanger 3 (HX3)    -   13. Heat exchanger 4 (HX4)    -   14. Heat exchanger 5 (HX5)    -   15. Heat exchanger at the first stage before going to the        thermal shield of the load (HX1stA)    -   16. Heat exchanger at the first stage after going to the thermal        shield of the load (HX1stB)    -   17. Heat exchanger at the 2nd Stage (HX2nd)    -   18. JT expansion device    -   19. Bypass-valve for Heat Exchanger 4 (HX4) at the feed side    -   20. Bypass-valve for Heat Exchanger 5 (HX5) at the feed side    -   21. Bypass valve for JT expansion device    -   22. Bypass-Valve for Heat Exchanger 4 and 5 (HX4 and HX5) at the        return side    -   23. Feed through connection for the transfer lines    -   24. Transfer line    -   25. Thermal Shield of the load    -   26. Pipe system for the thermal shield of the load    -   27. Load    -   28. Load pipe system    -   29. Thermal shield of the transfer line

1. A refrigeration system (1) for remote cooling of a thermal loadhaving a first portion (27) and a second portion (25), the systemcomprising: a cold source (4) having a first cooling stage (5) and asecond cooling stage (6), the temperature of the first cooling stagebeing higher than the temperature of the second cooling stage; a cryogencircuit for circulation of a cryogen flow in a closed cycle, the closedcycle being thermally coupled to the cold source; and a compressor (7)for compressing and circulating the cryogen flow in the cryogen circuit,wherein the cryogen circuit comprises a first conduit for thermallyconnecting the first cooling stage of the cold source to the firstportion of the thermal load so as to cool said first portion towards thetemperature of the first cooling stage, and a second conduit forthermally connecting the second cooling stage of the cold source to thesecond portion of the thermal load so as to cool said second portiontowards the temperature of the second cooling stage, and wherein thecryogen flow is a sub-cooled or saturated liquid, two phase, saturatedor overheated, supercritical gas helium flow.
 2. A refrigeration system(1) according to claim 1, wherein the first portion (27) of the thermalload is a thermal shield.
 3. A refrigeration system (1) according toclaim 1, wherein the second portion (25) of the thermal load is asuperconducting magnet.
 4. A refrigeration system (1) according to claim1, wherein the system further comprises a transfer line (24) in whichone or both of the conduits are located, and wherein the transfer linehas low thermal loss.
 5. A refrigeration system (1) according to claim1, wherein the first (5) and second (6) cooling stages and the first andsecond conduits of the closed cycle cryogen circuit are all connected inseries.
 6. A refrigeration system (1) according to claim 1, wherein thecold source is a cryocooler.
 7. A refrigeration system (1) according toclaim 1, wherein the cold source is contained in a cryostat that isseparate and independent from a cryostat of the thermal load.
 8. Arefrigeration system (1) according to claim 7, wherein the cryostatcomprises an actively cooled thermal shield (3).
 9. A refrigerationsystem (1) according to claim 1, wherein the system further comprises atleast one heat exchanger.
 10. A refrigeration system (1) according toclaim 1, wherein the system comprises means for performing aJoule-Thompson expansion step (18).
 11. A refrigeration system (1)according to claim 1, wherein the system can also act as a liquefier.12. A method for cooling remotely a thermal load using a refrigerationsystem (1), the method comprising: selecting the temperature of a firstcooling stage (5) of a cold source (4) of the system to be higher thanthe temperature of a second cooling stage (6); circulating a cryogenflow in a closed cycle around the cryogen circuit of the refrigerationsystem, the closed cycle being thermally coupled to the cold source, thecryogen circuit comprising a first conduit for thermally connecting thefirst cooling stage of the cold source to a first portion (27) of thethermal load so as to cool said first portion towards the temperature ofthe first cooling stage, and a second conduit for thermally connectingthe second cooling stage of the cold source to a second portion (25) ofthe thermal load so as to cool said second portion towards thetemperature of the second cooling stage; and compressing the flow in thecryogen circuit using a compressor (7) of the refrigeration system,wherein the cryogen flow is a sub-cooled or saturated liquid, two phase,saturated or overheated, supercritical gas helium flow.